In optical communication systems, increases in the transmission capacity and transmission distance of optical communication have been achieved by using wavelength multiplexing (WDM) technology and optical amplification relay technology. Furthermore, research and development is also being carried out into optical communication systems in which multicore optical fibers having a plurality of cores formed in each optical fiber are used in optical transmission paths in order to achieve further increases in transmission capacity (for example, see Japanese Laid-open Patent Publication No. 2009-251376, Japanese Laid-open Patent Publication No. 2002-338308, Japanese Laid-open Patent Publication No. 2013-235139, and International Publication Pamphlet No. WO 2011/116075).
In an optical fiber used as an optical transmission medium or an optical amplification medium, there are cases where the refractive index (may be referred to as the “refractive index profile”) between a core and a cladding, for example, is controlled in order to improve the effect of trapping light that is to be transmitted or amplified.
However, there is a limit to the light trapping effect that is able to be obtained with refractive index control. Therefore, loss may occur in light that propagates through the optical fiber. Furthermore, when refractive index control is to be carried out in a multicore optical fiber, there are cases where the optical fiber structure itself becomes complex and there is no choice but to increase the optical fiber diameter.
In one aspect, one objective of the present disclosure is to improve the effect of trapping multimode light that propagates through an optical transmission medium, within the optical transmission medium without resorting to refractive index control, to achieve an improvement in performance with regard to optical transmission and optical amplification for which the optical transmission medium is used.
Furthermore, in one aspect, an optical amplifier includes: the optical transmission medium; a multimode excitation light source that outputs multimode excitation light that is the aforementioned multimode light; and an excitation light introducing structure that introduces the multimode excitation light that is output from the multimode excitation light source, to a region inside a reflection layer of the optical transmission medium.